A Smashable refers to a very common item of various appearances (bottles, mugs, or glass cups) that can be thrown to make noise, which will attract the monster to the location of the sound. Smashables stack up to three in a single inventory slot, and are always destroyed when used.
Smashables are useful for luring a monster into a position where you can safely bypass it. They are also useful for pulling the monster closer to you, so that you can at least "trail" them instead of wandering blindly when they're in the area.
Do not pick up a smashable unless you intend to use it later; they cannot be dropped from your inventory without breaking and making noise.
To get rid of a smashable, it is advised to throw it into the ocean. The noise made will not call the monster and the item will just respawn where you found it.